A vehicle such as an automobile generally includes a seat belt device that restrains an occupant in a seat including a seat portion, on which an occupant is seated, and a backrest portion located on the back of the occupant. Such a seat belt device includes a webbing that restrains the occupant, a seat belt retractor that retracts the webbing, a guide anchor that is disposed on a vehicle body and that guides the webbing, a belt anchor that fixes the webbing to the vehicle body, a buckle disposed on a side surface of the seat, and a tongue disposed on the webbing. Fitting the tongue into the buckle causes the webbing to restrain the occupant in the seat.
Such a seat belt retractor often includes a spool on which a webbing is wound, a base frame that accommodates the spool while allowing the spool to rotate, a spring unit that applies a retracting force to the spool, a vehicle sensor (also referred to as an acceleration sensor or a shock sensor) that detects sudden deceleration of the vehicle, a locking mechanism that is activated by the vehicle sensor and that causes the spool to be engaged with the base frame, and a pretensioner that removes the slack of the webbing in case of emergency such as a collision of the vehicle (see, for example, Japanese Unexamined Patent Application Publication No. 2013-86759 and Japanese Unexamined Patent Application Publication No. 2014-80121).